The present invention relates to surface agitators for aerating liquids, and particularly to agitators of the type useful in the treatment of sewage and other waste water streams.
Mass transfer between gases and liquids is a very fundamental physical mechanism, similar in principle to the flow of electricity and the flow of heat. In chemical processes, mass transfer between gases and liquids can simply involve the physical transfer of one material into the other material up to the saturation value, at which point transfer stops. Or, it can also include a chemical reaction occurring between the dissolved material in the second material, after the basic mass transfer has occurred. In such a case, mass transfer will continue until no more reaction occurs.
An extremely important mass transfer is involved in the treatment of sewage and other wastewater streams. The basic process is known as the activated sludge process, which was discovered or invented about fifty years ago. It is a biochemical type of reaction, involving the mass transfer of oxygen to water, and then the transfer of that dissolved oxygen to support the growth of a microorganism being suspended in water. These microorganisms, known as the biomass, work on the waste stream in different ways, depending on what it is, to eliminate the so-called bacteriological oxygen demand (BOD).
The original activated sludge process involved introducing air from a blower through various forms of diffuser devices located in the bottom of the basin. These devices generally have low oxygen-transfer efficiency and poor solids-suspension characteristics.
About twenty-five years ago, a different approach was taken to aeration in the activated sludge proces. This different approach was known as mechanical surface aeration. This consisted of a mechanical agitator operating at the liquid surface to throw liquid into the air and to induce absorption of air into the liquid, without the use of a compressor and the diffusers. Since that time, a fairly large number of different designs for surface aerator impellers have been introduced, both for the purpose of increasing the oxygen-transfer efficiency and also, secondarily, if possible, to improve the solids suspension. The problem of solids suspension, however, has an obvious limitation because of the remoteness of the impeller from the tank bottom where the solids tend to accumulate.
The oxygen-transfer efficiency for surface aerator mechanisms ranges from 2.0 to 4.0 pounds of oxygen per hour per horsepower in the larger sizes. In the smaller sizes, the efficiency values are usually higher. Since electricity is one of the main operating costs in such a plant, the oxygen-transfer efficiency value is extremely important.
Over the years, surface aerators in general have unfortunately been plagued with an unusual amount of mechanical breakdown problems. In general, these have been due to erratic loading characteristics of the various types of surface aerator impellers on the market, which cause unusually high shock loads through the power transmission train. These have generally resulted in gear failures, although other failures, such as bearing failures and shaft failures, have also occurred. Some surface aerator impellers are characterized by instantaneous power load swings of 25 to 50 percent. These are apparently due to a combination of collapsing vortexes, plus the phenomenon of the blade jumping from liquid to gas and back to liquid again because of close proximity to a violently agitated liquid surface. Aerator developers have attempted to create an aerator design which would give a more uniform transmission of load and avoid failures due to shock loading, but generally the efforts have not been very successful.
Another major problem associated with surface aerators is the problem of misting and subsequent ice formation in cold climates. It has been noted that some surface aerators create more mist than others, and the subsequent formation of ice can result in serious damage to the equipment. In addition, more recently, concern has developed about this misting resulting in aerosol type transport of harmful bacteria to neighborhoods adjacent to the sewage treatment plant. Currently, the question of misting is becoming of increasing concern regardless of the nature of the climate where the plant is located. Surface aerator manufacturers and consulting engineers have attempted to control misting by the use of shields located above the plume of the impeller. However, this does not directly control the formation of mist; it simply diverts that mist from impingement on the immediate platform area.
Another operating difficulty with some surface aerator designs is the problem of large floating material, such as rags, getting caught on the complicated impeller configurations and creating mechanical unbalance leading to mechanical failures. Unfortunately, in general, those surface aerator impellers which have a high oxygen-transfer efficiency are also of very complex design and are prone to this kind of problem.